If you love the 5'8" Spitfire (31 litres) I'd have thought the 5'10" Hellfire (29.4) would be the next board up, and maybe a 6'1" (29.1) or 6'3" Hellrazor (30.9) would be the go, probably the 6'3" in the HR to diferentiate it more from the other two.

I'm 10kg on you in lesser waves and surf a 6'4" Dom, but could surf a 6'2", particularly if it was in consistently better waves. Drawing a comparison from that I'd have thought you'd be on a 5'10" Spitfire (33.5l), and it would therefore follow you'd be on a 6'0" Hellfire (31.9l), and a 6'3" or 6'5" Hellrazor (33.5).

All in all if the Spit works for you at the 5'8" length then I'd be suggesting to go with the suggestions in the first paragraph.

I see Chris has mentioned in another thread you go the 6'4" Hellfire, which is the length I'd be looking at @90kg. It really depends on how much ease of paddling you are after, your ability, and how clean/lined up the waves you are surfing. I personally think at 80kg if you love the Spit at 5'8" the 6'4" Hellfire will feel way overfoamed, but then again (as intimated above) I have an inkling you might be riding an underfoamed Spit for your weight, particularly if you're looking for paddling assistance.